1. Field of the Invention
The present invention relates to an image reading device and an image forming apparatus including the image reading device. More specifically, the present invention relates to an image reading device including a plurality of image sensors and an image forming apparatus that includes the above-described image reading device.
2. Discussion of the Related Art
A related art image reading device used for a digital copier, printer, facsimile machine, and so forth can include a linear CCD (charge coupled device) image sensor. The linear CCD image sensor can include its elemental devices for reading an image.
The linear CCD image sensor can electrically move in a main scanning direction of the image or an axial direction of an image forming component such as a photoconductor, and read the image by using an elemental device thereof. The linear CCD image sensor can also mechanically move in a sub-scanning direction of the image or a direction perpendicular to the axial direction, and read the image.
Such an image reading device can employ a dual-output type CCD image sensor or a quad-output type CCD image sensor to increase a speed for reading an image. Elemental devices of the CCD image sensor may have respectively different responses with respect to sensitivity and dark current. Further, a dual-output type CCD image sensor and a quad-output type CCD image sensor may have differences in amplifiers of an output part of each output channel, analog circuits of an analog signal processing part, and characteristics of an analog-to-digital conversion part, and so forth.
When a shading correction is conducted, concentrations of white level and black level can be corrected to respective given levels. However, the output characteristics may be different in pixels. This may cause variations or non-uniformity in grey level. Such variations or non-uniformity in grey level may produce an output image having non-uniformity, such as asymmetric concentration or streaks in a vertical direction or a sheet moving direction.
The output characteristic may represent a brightness of an original document, specifically, a relationship of an output value of a CCD image sensor with respect to an input value of the CCD image sensor. For authentically reproducing an image of an original document, it is preferable an output characteristic may show a linear form.
Techniques of related art image reading devices have proposed to use correction LUTs (look up tables) to correct difference of the characteristics of output channels.
According to one of the above-described techniques, a related art image reading device having a dual-output type CCD image sensor uses one correction LUT with respect to pixels having even numbers, and a different correction LUT with respect to pixels having odd numbers.
According to a different one of the above-described techniques, a related art image reading device having a quad-output type CCD image sensor uses different LUTs for the leading end data and trailing end data of one line data. By so doing, differences between the first half in a main scanning direction of image data and the second half in the main scanning direction of the image data may be corrected.
Related art image reading devices for handling large-sized sheets generally include a contact image sensor for reading image data while a large-sized original document is moving along a sheet moving direction. It is not preferable and practical to move an optical device over a large-sized original document since a large table for placing the original document may be needed and a carriage for moving the optical device may need to be made to fit a large-sized document.
A contact image sensor included in an image reading device that can handle both a regular-sized and large-sized sheets may need to have a width or distance in a main scanning direction greater than or equal to the width of a large-sized original document. For example, for reading an A0 size original document, the width of a contact image sensor may need to be greater than or equal to 841 mm. In this case, one contact image sensor may not be technically sufficient. In addition, the cost for the contact image sensor may increase.
To avoid such insufficiency in technique and increase in cost, a plurality of small contact image sensors may be aligned in the main scanning direction. The plurality of small contact image sensors may receive light beams emitted thereto while reading an original document, form an image of the original document thereon, and electrically connect image signals read by the plurality of small contact image sensors. By conducting the above-described operation, image data corresponding to the entire scanning lines may be obtained.
In such image reading device including a plurality of image sensors, as described above, each of the image sensors may have respectively different responses with respect to sensitivity, and have difference in amplifier of an output part of each output channel, analog circuit of an analog signal processing part, and characteristic of an analog-to-digital conversion part, and so forth.
Therefore, when a shading correction is conducted, concentrations of white level and black level may be corrected to respective given levels. However, the output characteristics may be different in pixels. This may cause variations or non-uniformity in grey level. Such variations or non-uniformity in grey level may produce an output image having non-uniformity and different concentrations.